In Third Generation Partnership Project (3GPP) standards, heterogeneous network deployments have been defined as deployments where base stations of different transmit powers are placed throughout a macro-cell layout. Examples of low-power base stations include micro, pico, and femto base stations. Heterogeneous network deployments provide capacity extension in certain areas such as traffic hotspots. A traffic hotspot is a small geographical area with a higher user density and/or higher traffic intensity than the surrounding area. Placing a low power base station such as a pico base station at a traffic hotspot can adapt a network according to traffic needs and environment, thus enhancing the network's performance. However, interference characteristics in a heterogeneous deployment are significantly different than in a homogeneous deployment, in both downlink and uplink. Also in a heterogeneous network deployment, traffic distribution is often non-uniform and uplink transmissions generally experience high interference due to the co-existence of both small and large cells.
It is often challenging to ensure efficient network operation and superior user experience in a heterogeneous network deployment. One common problem related to heterogeneous networks is that it is often difficult for a wireless device to perform measurements on signals transmitted from a low-power base station in a heterogeneous network due to interference from neighboring high-power base stations. Yet accurate signal estimates and measurements are needed for important functionalities such as cell search, cell identification, handover, Radio Link Management (RLM) and Radio Resource Management (RRM), etc. The difficulty of the measurements is compounded in scenarios where cells providing overlapping coverage operate on different carrier frequencies.